Photoplethysmographic measurement instruments are configured to emit light of at least two different, predetermined wavelengths through a selected portion of a patient's anatomy (e.g., a finger tip. ear, nasal septum). The analytes to be identified within the patient's blood must each have unique light absorbency characteristics for at least two of the emitted wavelengths. By measuring changes in intensity of the transmitted (the light exiting an absorber is referred to as transmitted) light from the patient's finger (or other suitable area of anatomy) at these wavelengths, each analyte such as reduced hemoglobin (RHb) and oxygenated hemoglobin (O.sub.2 Hb) may be determined. Thereafter, characteristics such as blood oxygen saturation (SpO.sub.2) may be determined based on these analytes. An example of this type of photoplethysmographic instrument which measures reduced hemoglobin (RHb) and oxygenated hemoglobin (O.sub.2 Hb), known as a pulse oxinmeter, can be found in U.S. Pat. No. 5,503,148 to Pologe et. al which is incorporated herein by reference.
Photoplethysmographic monitors have also been developed which use LED's or laser diodes to measure one or more additional blood analytes levels, such as carboxyhemoglobin and methemoglobin, in addition to RHb and 02Hb, based on similar principles to pulse oximetry described above.
Other characteristics such as pulse rate may also be determined based on certain components of the transmitted light signal which passes through the patient's anatomy. Specifically, the transmitted light includes a large DC component and a smaller AC or pulsatile component. By using the pulsatile component, the patients pulse rate may be determined, since fluctuations in the pulsatile component are a function of arterioles pulsating with the patient's heart rate.
In one photoplethysmographic measurement system known as a pulse oximeter, at least two wavelengths of light may be emitted during dedicated, alternating intervals. The transmitted light from the selected body portion is detected by a light-sensitive element such as a photodiode. The photodiode then outputs a time division multiplexed (TDM) signal that includes portions corresponding with each wavelength of the transmitted light
For example in a TDM pulse oximeter, each emitted light level can be immediately preceded by an ambient light interval which may also be referred to as a "dark time" interval. The system first de-multiplexes the TDM signal into parallel channels. Signal processing then proceeds wherein a first series of steps performs preliminary filtering. Immediately following the first series of steps, the parallel channels are re-multiplexed. Next, a second series of steps is performed in which the re-multiplexed signal facilitates subtraction of the dark time signal from the signal corresponding with each emitted light interval in a manner known in the art. Such subtraction process may rely on a dark time interval immediately preceding each emitted light interval or at least one of the light intervals in a TDM format. Following the second series of steps, in which ambient light subtraction is accomplished, the TDM signal is de-multiplexed a second time into parallel channels prior to the completion of signal processing.
The photodiodes used as the light sensitive elements in pulse oximeters do not turn off instantaneously after the excitation of the emitters, and, therefore the transmitted light incident on the photodiode, ceases. In other words, the received light signal coming from the photodiode as a result of the emitted light being transmitted through the tissue of the patient may take more than 100 microseconds to decay from 10% to 0.1% of the maximum received light signal intensity. This compares to a decay time of perhaps only several microseconds for the decay from 90% to 10% of the maximum received light signal intensity. This slow turn-off of photodiodes for the last 10% of decay can erroneously become a portion of the integrated ambient light level of the following channel. Because the light level used for calculations for any channel is simply the difference between the channel's light time and dark time, an error in the dark time is essentially the same as an error in that channel's light time.
The problems associated with the slow turn-off of photodiodes are also exacerbated at longer wavelengths and when a large gain disparity exists between the two channels (e.g., the gain is greater for the second channel). As the gain disparity increases so does the error caused by the undecayed signal which bleeds through the photodiode into the ambient light measurement.